The present invention relates to a friction unit for frictional engagement with a counter-body, especially a brake or clutch body, having at least one freely accessible friction surface which is formed of a carbon fiber-reinforced, porous carbon body whose pores are at least partially filled with silicon and silicon carbide.
The invention furthermore relates to a method for manufacturing a friction unit for frictional engagement with a counter-body, especially brake or clutch bodies, wherein a carbon fiber reinforced, porous carbon body is prepared and infiltrated with fluid silicon at a temperature in the range from 1410.degree. C. to 1700.degree. C. in a controlled atmosphere.
Such friction units and method for their manufacture were presented by a work group of the DLR (Deutsche Forschungsanstalt fur Luft- und Raumfahrt e.V.), Stuttgart, Institut fur Bauweisen- und Konstruktionsforschung, at the 1994 VDI Materials Convention in Duisburg on Mar. 9-10, 1994, the theme of which was lightweight structures and lightweight components, in the framework of the lecture entitled "Development of integral lightweight structures of ceramic fiber." In this lecture, a technology for manufacturing carbon fiber-reinforced carbons was presented. The carbon fiber-reinforced carbons are infiltrated with fluid silicon by a so-called "fluid silicification method" and subjected to a heat treatment, wherein the silicon is converted with carbon to SiC. One possible application of these C/C--SiC materials is, among others, brake disks.
Increasingly severe requirements are being made of brakes, especially in motor vehicle and aircraft construction. The speeds which are attained nowadays by such vehicles are constantly increasing. When brakes are applied, the kinetic energy is converted by friction to heat, which is absorbed by the brake disk and the brake linings. A brake system of this kind is accordingly limited by the friction characteristics of the brake material and its ability to store and remove heat. In general, brake materials must have very good thermomechanical properties, high and constant friction characteristics and good resistance to abrasion. Ordinary brake disks of cast iron or steel, which are today used in ordinary automobile construction, permit temperatures of approximately 650.degree. C. Brakes of carbon fiber-reinforced carbon materials (C/C) developed in recent years, such as those described for example in DE-A1 3 24 200, allow temperatures up to 100.degree. C., combined with a weight reduction in comparison with cast-iron brake disks.
Such carbon brake disks have in the meantime been adopted and established in race car construction and aircraft construction. What is problematic in this case is, in addition to a number of tribological properties, the high cost factor involved in the brake disks, which presently is accepted in the field of race car and aircraft construction, but cannot be accepted for general vehicle construction.
At the above-described VDI presentation, a C/C--SiC material was exhibited, as stated above, which shows decided advantages over a C/C material, especially in regard to thermal shock resistance, oxidation resistance, moisture absorption and frictional performance.
Setting out from the state of the art described above, the present invention is addressed to the problem of devising a friction unit as well as a method for the manufacture of such a friction unit, in conjunction with which the advantages are achieved which are associated with C/C--SiC materials, which can be manufactured at a reasonable cost, so that such friction units are economically feasible especially for general vehicle construction.
The above-stated object is obtained by a friction unit formed of at least a core body and at least a friction body fixedly joined thereto, the friction body being joined to the core body on its side facing away from the friction surface, and both bodies being joined together as intimately as possible by a refractory bonding layer.
In another aspect, the invention is in a method wherein at least one additional body is prepared which is joined to the silicon-infiltrated body by a refractory bonding layer, the carbon body forming a friction body and the other body forming a core body.
The basic idea of the invention lies in making such a friction unit of a plurality of parts, so that the individual areas of a friction unit can be adapted in use to particular requirements. A friction unit of this kind is divided into at least one core body and at least one friction body, the latter being formed of carbon fiber-reinforced, porous carbon bodies whose pores are at least partially filled with silicon and silicon carbide, and these two bodies are joined together in a single unit. The friction body can be adapted to desired requirements in its quality, i.e., its friction coefficient and its abrasion resistance. The core body, however, is constructed such that it is suitable on the one hand as a support for the friction body, i.e., it has a high mechanical stability, can still contain accommodating parts, adapter parts and joining parts in order to secure it on a stationary or rotating unit, and lastly it is adapted in its material properties such that it absorbs the heat well and quickly dissipates it. Furthermore, this multi-partite bonded construction offers the possibility of joining the remanent core body to a fresh friction body. The reuse of the core body considerably reduces the cost of such a friction unit by the simple replacement of the worn-out, abraded part. Furthermore, the costs can be reduced in mass production by subdividing a friction unit into the individual bodies, i.e., core body and friction body, with the adapted material properties, especially if such friction units have to be textured. For this purpose no complicated after-working procedures are necessary, since the parts of the friction unit, i.e., the core body and the friction body, are produced and shaped individually and only then are they joined together at a common, smooth or possibly toothed joining surface. The technique of producing C/C--SiC bodies, which will be explained hereinafter, makes it possible to preform and profile such parts in a "green" state, then infiltrate with fluid silicon, and ceramicize the part in a heat treatment. The compacts are very easy to shape while they are in the green state. For example the core body can be provided with appropriate shapes for fastening, and holding means can be incorporated into these core bodies. In contrast, the friction coating, in its simplest version, can be made in the form of a circular disk smooth on both sides. Any ventilation openings within such a friction unit can be incorporated either into the face of the core body that is later to be joined to the friction body, or else it can be incorporated in the junction surface of the friction body. Therefore, in the present invention, a simple functional separation of the expensive friction volume and the inexpensive core volume is achieved. Furthermore, the invention permits optimization of the mechanical and thermal properties associated with the friction volume and the core volume. It has been found that, in order to join the two bodies, i.e., the core body with the friction body, it is not necessary to use dissimilar materials. Instead, a joining layer is used which contains substantially silicon carbide. There are different possibilities for joining the two bodies. In one aspect, in the manufacture of a new friction unit from a core body and at least one friction body, the two bodies can advantageously be prepared as carbon fiber-reinforced, porous carbon bodies and then infiltrated with silicon, and these two bodies are joined together at the joining layer after heat treatment. Another possibility is for the two bodies to be prepared in the form of ceramicized starting bodies already filled with silicon and silicon carbide. These two starting bodies are placed one on the other at their joining surfaces and the gap is filled with silicon. For this purpose a technique is preferred in which this gap is infiltrated with silicon and then the entire unit is subjected to a heat treatment using temperatures ranging from 1410.degree. C. to 1700.degree. C. Such a technique of bonding two finished bodies is to be used whenever a worn-out friction unit, in which only a thin layer of the friction body remains, is renovated with a new friction body. To promote the infiltration of the fluid silicon in the area of the bonding layer, an insert of porous, pyrolyzable material on a cellulose basis can be interposed between the friction body and the core body prior to the infiltrations. Preferably this insert consists of paper, cardboard and/or paper felt with a high porosity, the thickness ranging preferably from 0.1 to 1 mm. Such an insert is then infiltrated with fluid silicon and then the prepared unit is subjected to the heat treatment. To accelerate the infiltration and render it more uniform in the insert, a pressure gradient can be produced and sustained during infiltration in the area of the layer being formed, for example by applying a vacuum.
A carbon fleece or a carbon mat can also be used as the insert. Preferably, with such an insert, any differences in the gaps in the junction area are evened out. The material on a cellulose basis is especially suitable for this purpose. An insert of carbon mat or of a fiber material containing carbon is when the direction of flow and velocity of flow in the gap are to be influenced. It is certainly to be stressed at this point that the two materials mentioned as insert react after infiltration with liquid silicon and the heat treatment to form SiC, and thus an insoluble bond is formed. Such a procedure for bonding the two bodies is to be selected whenever a friction unit is to be provided at some time with a new friction body without completely removing the old friction body in any way.
Normally, no insert is used, but only a defined gap of 0.1 to 0.5 mm is left between the friction body and the core body. Fluid silicon is introduced into this gap and ceramicized. In the ceramicized state the bonding layer then contains substantially silicon, which can be liquefied by reheating to the melting temperature (1420.degree. C. for silicon), so that the friction body and core body can be separated again from one another. The silicon serves in this case as a hard solder.
From the above description it can be seen that, with the procedure described, repeatedly built-up or multi-layer friction units can be made, and a core body can be joined to a friction body on two opposite sides by means of a bonding layer containing mostly silicon carbide on each. Furthermore, core bodies and friction bodies can be joined alternately with one another in order to form a multi-layer friction unit, the individual friction bodies protruding past the circumference of the core body, so that the friction surfaces of the friction bodies can be accessible from the exterior. With this modular-like build-up it is necessary only to stock friction bodies and core bodies in order then to join them together in the desired order. The friction bodies can be of the variety which subsequently can be applied to a core body when the original friction body is worn out.
The core body should preferably have a porosity of 5 to 50%, and especially a porosity ranging from 10 to 30%. These pores are then infiltrated with silicon which is converted by heat treatment to silicon carbide. The residual porosity should amount to less than 10% in order to make this core body mechanically stable yet at the same time elastic enough to satisfy requirements as supports when used as a clutch or brake unit.
To increase the thermal conductivity of the friction body and/or core body, care must be taken that carbon fibers are present in the direction of the thickness in the amount of 3 to 10% of the total fiber content. This can be achieved by using three dimensional fiber skeletons or by sewing together two-dimensional fabrics with carbon sewing threads.
In order to adapt the friction body to its requirements in use, additives reducing or increasing friction are embedded in the carbon fiber-reinforced, porous carbon bodies. For example, boron nitride and/or aluminum phosphate are embedded into the pores as friction reducing additives, while a silicon carbide powder with a grain size of 0.3 to 3.0 .mu.m, for example, is embedded as a friction increasing additive. The friction coefficient is increased by additives increasing the friction value. The friction increasing additive in the form of silicon carbide powder has the advantage that this powder can be embedded in the friction body at particular points at which an increased friction value is desired.
Of course, it has been found that friction bodies as described above tend to be very noisy, i.e., to squeal, under certain conditions of their use. Such noise is not acceptable in passenger car construction. For this reason the embedding of friction reducing additives in the form of the above-mentioned boron nitride and/or aluminum phosphate is helpful, making it possible to prevent such squealing.
It is also conceivable for the friction body to contain different additives in different places, i.e., a friction-increasing additive in one area and a friction reducing additive in another, for example in consideration of the different angular velocities of different areas of a disk-shaped, rotating friction body.
The core body can be made most economically of porous carbon or at least partially of carbon fibers. By the use of these materials in the core body the costs of the core bodies can be reduced. If the entire core body is formed of carbon fibers, the individual layers of the fibers can be stacked one on the other or wound, and the orientation of the fibers in adjacent layers can be different, oriented or unoriented. Preferably the fiber length is between 1 and 10 mm, so that a preliminary body will result, having a well-crosslinked, porous structure, which is then infiltrated with silicon. On the other hand, the core body can also be formed of silicon carbide or a mixture of silicon carbide and graphite. In the form of silicon carbide, a cheap material is used, which furthermore has a high thermal conductivity and accordingly satisfies the requirements of a core body.
Preferably, the content of the bonding layer of silicon carbide in the finished friction unit is above 50%. Thus a good heat transfer is achieved between the friction unit in which the heat is produced and the core body which is to store and remove the heat.
It is a preferred embodiment to form a bonding layer which has a slurry added which consists of an organic binding agent with a residual carbon content of at least 40% and a fine powder of carbon and/or silicon, the binding agent content amounting to between 10 and 50%. Phenol, for example, can be used as the binding agent. This allows the amount of carbon powder added to be kept low while obtaining a high yield of SiC.
The production of a C/C--SiC body can be summarized briefly as follows:
First a pore-free and homogeneous carbon fiber-reinforced carbon body is produced as a preliminary body, consisting of carbon-rich polymers as matrices and endless fibers. In the second production step the thermal conversion of the matrix to glasslike carbon is performed by pyrolysis, which results in a carbon fiber-reinforced, porous carbon body with translaminar channels. In the third production step fluid silicon is infiltrated into the pores, and reacts under heat with the carbon of the matrix to form silicon carbide. All production steps are performed only once, in contrast to other known processes. The result is a dense structure consisting of high-strength carbon fiber clusters and oxidation-inhibiting silicon carbide protective layers surrounding the fiber clusters.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.